What size tank do you have? And what is the filter rated for? I know goldfish need 2x the amount of filtration then the size of the tank. That should have the water circulating fast enough to get rid of the food.

I have a 29 gallon tank. My HOB filter is rated for 30 gallons, and I have a homemade sponge filter which features chemical, mechanical, and biological filtration which is worth at least another 20 to 25 gallons. I just don't have it plugged in at the moment because its night and it agitates the water a bit, and the bubble sound annoys me.

Gel food is great for goldfish, but I think you've gone about it in the wrong manner. It's best to follow recipes that have already been made by experienced and knowledgeable goldfish keepers rather than make your own. Oatmeal should not be used as it is a grain and can cause bloating issues in fancies. Your source of protein should be fresh fish and not pellets as all pellets contain wheat which is also bad for the goldfish GI tract. Gel Food Recipes This has some gel food recipes that require a blender, but I prefer babyfood gel food recipes. This is the one I use from the GAB:

Boil the water, add the gelatin stir till dissolved.
Add the baby food and the salmon.
Dissolve the multi vitamin in a tablespoon of water, add.
Refrigerate till set then cut into cubes.
You can add a half cup or so of finely chopped veggies to this mix as well.

It's easiest to hand-feed goldfish gel food. That way you can regulate who gets what and how much. It also prevents food from falling on the bottom. I do not recommend getting an algae eater.

Your tank size is entirely too small for a common goldfish. Even with a strong filter (your filter on that tank should be rated for a 55 gal and up), it will not provide enough swimming room for a fully grown common goldfish. Common goldfish should be housed in a 55 gal minimum. A 75 gal or larger is better. And a pond is best. A 29 gal is suitable for two smaller fancy goldfish, but even then I don't recommend this size tank for goldfish. A 40 gal as a min is best.